262 THE AMERICAN MONTHLY [November, 



and theory during two years without finding a solution. The chief 

 purpose of this paper is to record a series of experiments by the repe- 

 tition of which any sufficienth' interested microscopist can satisfy him- 

 self that Dr. Blackham's demonstration was in some way at fault, that 

 a real image cannot be formed under the prescribed conditions, and that 

 the failure to theoretically explain the formation of an image under 

 those conditions was a proper and to be expected result. 



Experiment i. — The rulings of a stage micrometer, under an inch 

 and a half objective, were focussed for a normal eye looking through a 

 horizontally arranged microscope. A screen was placed in contact with 

 the eye-piece and slowly removed to a distance of five feet without find- 

 ing an image in an}' plane through which it passed. But so soon as the 

 microscope was racked slightly awa}' from the object, a sharp image 

 appeared on the screen, and on again looking through the microscope, 

 the lines seen at first had disappeared. 



Experiment 3. — The second experiment was a repetition of the first, 

 excepting that instead of micrometer rulings approximately lying in a 

 single plane, the object was a somewhat thick section of lung tissue. 

 When the object was in focus for a normal eye looking through the 

 microscope, a blurred image could be seen on the sci^een. On racking 

 the microscope away ffom the object, the image on the screen became 

 sharp, while the object, as seen through the instrument, lost its sharp- 

 ness. In the former instance, the sharp focus was found to be on the 

 nearer surface of the object, while the blurred image was of a deeper 

 plane ; and in the second instance, vice versa. 



Experiment 3. — The dust on the cover-glass of a mount, under an 

 inch and a half objective, was focussed for a normal eye looking through 

 the microscope. On removing the head, an image of the object beneath 

 the cover-glass appeared on the screen about ten inches from the eye- 

 piece. 



Experiment 4. — A nummulite, under a three-inch objective and two- 

 inch eye-piece, was focussed for a normal eye looking through the mi- 

 croscope. With this arrangement a sensitized plate was exposed at ten 

 inches from the eye-piece. Photo-micrograph i is a print from the result- 

 ing negative. The result is as good as could be obtained at any distance 

 from the eye-piece under the prescribed conditions. The microscope 

 was then racked away from the object until a sharp image appeared at 

 the ten-inch distance. A second sensitized plate was exposed at ten 

 inches from the eye-piece. Photo-micrograph 2 is a print from the 

 resulting negative. 



Experiment ^. — A physician, 68 years old, with presbyopic eyes and 

 long accustomed to the use of a microscope, focussed — ^without his spec- 

 tacles — an object as he saw it through the e}'e-piece. On removing his 

 head, an image of the object appeared on a screen about twenty-seven 

 inches from the eye-piece. A normal eye looking through the micro- 

 scope could not see the object. , 



Experiment6. — An Abbe test-plate, under an inch and a half objective, 

 was focussed for an eye made, as it were, temporarily hypermetropic 

 by wearing a six-inch negative spectacle lens. On removing the head, 

 a sharp image appeared on a screen about ten inches from the eye-piece. 

 The normal eye, without the spectacle lens, could not see the object on 

 again looking through the microscope. 



